Drive with voice coil motor

ABSTRACT

A drive is provided executing high-speed oscillatory motion. A voice coil motor in a pixel shifter includes a coil of a rectangular shape producing thrust along an axis, a flap fixing the coil, and flat springs giving to the flap an urging force against the thrust. An oscillation axis of the flap is parallel to the axis and located between another axis that is parallel and opposite to the axis and still another axis that is parallel to the axis and passes over a centroid of the coil. This reduces the value of the inertia per unit torque.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a drive with a voice coil motor thatexecutes oscillatory motion.

2. Description of the Background Art

Recent projection image displays that project images on a screen bylight projection employ a device (hereinafter referred to as a “pixelshifter”) that allows incident light to pass through glass so that theglass oscillates slightly, thereby shifting the incident light in thedirection of oscillation. Shifting an image on a screen by half a pixelwith this pixel shifter has the effect of increasing the apparent numberof pixels, thereby improving resolution.

The actuator used in this case is a voice coil motor that includes acylindrical coil, a cup-shaped outer yoke provided to cover the outerside of the coil, a permanent magnet provided at the center of the innerface of the outer yoke and on the inner side of the coil, and an inneryoke provided at the top end of the permanent magnet.

The device that executes oscillatory motion is also a cylindrical voicecoil motor. Its center of oscillation is located outside the voice coilmotor and at about the midpoint position between a position where theoscillatory motion occurs and the voice-coil motor.

Examples of an apparatus using a voice coil motor as an actuator aredisclosed for example in Japanese Patent Application Laid-open No.5-326603 (cf. paragraph [0002] and FIG. 1) and the like.

A cylindrical voice coil motor generates a thrust throughout theperiphery of a coil, thus achieving efficiency; however, it has theproblem of difficulty in machining parts such as a cup-shaped yoke.Thus, in some cases, a voice coil motor of a different shape is used inwhich a yoke (center yoke) of a metal plate is provided in a hole at thecenter of a rectangularly-wound coil, and one magnet is provided in aposition to sandwich one side of the coil. In this case, an end face ofanother yoke (back yoke) provided at the rear of the magnet and an endface of the center yoke provided in a hole at the center of the coil aremade in contact with each other, which provides a structure that allowsthe magnetic flux from the magnet to circulate, thus improving theefficiency of the voice coil motor.

When the aforementioned voice coil motor with a rectangularly-wound coilis adopted, one side of the coil that is the furthest away from thecenter of oscillation is used as a voice coil motor in order to increasetorque. As the one side of the coil used as a voice coil motor isfurther away from the center of oscillation, higher torque can beobtained with only a small thrust.

However, since a coil is mainly made of copper and thus concerned as oneof the oscillating parts of relatively high mass, its increased distancefrom the center of oscillation will undesirably increase the inertia ofthe coil, making it difficult to increase the speed of oscillation.

SUMMARY OF THE INVENTION

The invention has been made in view of the aforementioned problems andhas the object of providing a drive that executes high-speed oscillatorymotion.

The drive according to the invention includes a voice coil motor. Thevoice coil motor includes a coil of a rectangular shape producing thruston its one side; an oscillating part fixing the coil; and an elasticbody giving to the oscillating part an urging force against the thrust.An oscillation axis, which is the center of oscillation of theoscillating part, is parallel to the one side and located between a coilface outside another side that is parallel and opposite to the one side,and a centroid axis that is parallel to the one side and passes over acentroid of the coil.

The drive according to the invention reduces the value of the inertiaper unit torque and accordingly lessens the influence of the inertia ofthe coil, thus executing high-speed oscillatory motion.

These and other objects, features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an external perspective view illustrating the structure of apixel shifter according to a first preferred embodiment;

FIG. 2 is an exploded perspective view illustrating the structure of thepixel shifter according to the first preferred embodiment;

FIG. 3 is an external perspective view illustrating the structure ofoscillating parts of the pixel shifter according to the first preferredembodiment;

FIGS. 4A and 4B illustrate the relationship between a coil and thecenter of oscillation in the pixel shifter according to the firstpreferred embodiment; and

FIG. 5 is a graph showing the relationship between the distance from aposition where the coil produces thrust to the center of oscillation andthe inertia per unit torque in the pixel shifter according to the firstpreferred embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, preferred embodiments of a drive according to the invention aredescribed with reference to the drawings.

First Preferred Embodiment

FIG. 1 is an external perspective view illustrating the structure of apixel shifter 10 according to a first preferred embodiment of theinvention, FIG. 2 is an exploded perspective view thereof, and FIG. 3 isan external perspective view of oscillating parts.

Referring to FIGS. 1 to 3, a flap 1 as an oscillating part has attachedthereto glass 3 passing through incident light, a rectangular coil 4 asa component of a voice coil motor, and flat springs 2 generating areaction force against the thrust of the voice coil motor. The T-shapedflat springs 2 are fixed at the center of oscillation of the flap 1 andgive an urging force so that the flap 1 abuts against supporting points7 a provided on a base 7. The glass 3 is provided on the opposite sideto the coil 4 across the axis of oscillation. The oscillatory motion ofthe flap 1 causes the flat springs 2 to generate a reaction force due totorsion at their portions 2 a and a reaction force due to deformation attheir portions 2 b.

The voice coil motor includes the coil 4 fixed to the flap 1, anL-shaped yoke 5 a passing through a hole 4 a that opens at the center ofthe coil 4, a yoke 5 b provided outside the coil 4, and two magnets 6attached the inner faces of the yokes 5 a and 5 b to sandwich the coil4. The L-shaped yokes 5 a and 5 b are made in contact with each other attheir end faces so that the magnetic lines of force from the north poleof the magnets 6 are circulated to the south pole.

Passing current through the coil 4 produces a thrust in the directionperpendicular to the magnetic lines of force passing in the verticaldirection of the drawing across the coil 4. This thrust causes the flap1 to oscillate on the supporting points 7 a against the spring force ofthe flat springs 2.

The parts fixed to and oscillating along with the flap 1 are the glass 3and the coil 4. Referring to the mass of each part, for example the flap1 has a mass of 1.5 g, the glass 3 has a mass of 0.7 g, and the coil 4has a mass of 2.1 g, which indicates that the mass of the coil 4constitutes about half the total mass of the oscillating parts. Thisapparently shows that reducing the inertia of the coil 4 is importantfor improvement in the response characteristics of the pixel shifter 10.

FIGS. 4A and 4B illustrate the relationship of the coil 4 and the centerof oscillation. FIG. 4A is a front view of the coil 4 and FIG. 4B is across-sectional view of the coil 4. The shape of the coil 4 is expressedby the external width b₁, external height h₁, internal width b₂, andinternal height h₂ of the coil 4. Let X be the center of oscillation,i.e., the oscillation axis; X₀ be the axis (centroid axis) parallel tothe oscillation axis X and passing over the centroid G; c be thedistance between the axis X₀ and an axis X₂ that passes over the centerof the part of the coil 4 where the thrust is produced; d be thethickness of the coil 4; t be the depth of the coil 4; and ρ be the massof the coil 4 per unit volume.

The inertia I of the coil 4 is expressed by the square of the product ofthe mass of each tiny part of the coil 4 and the distance thereof fromthe center of oscillation.

The mass of each tiny part dy of the coil 4 is given by the followingequation (1).dM=ρ·t·dy·d  (1)

The inertia I of a tiny part dy in the drawing is thus given by thefollowing equation (2), using the distance y′ between the tiny part dyand the oscillation axis X.I=ρt·dy·d·y′ ²  (2)

By substituting each parameter shown in FIGS. 4A and 4B into equation(2) and using a well-known formula for calculation of the geometricalmoment of inertia, the inertia I of the coil 4 with the oscillation axisX as the center is given by the following equation (3), using thedistance y between the axis X₀ and the oscillation axis X.I=ρ·t(b ₁ ·h ₁ −b ₂ ·h ₂)y ² +ρ·t(b ₁ ·h ₁ ³ −b ₂ ·h ₂ ³)/12  (3)

When thrust F is produced on the bottom side of the coil 4, the torque Thaving the oscillation axis X as its center is given by the followingequation (4).T=F·(y+c)  (4)

Since the inertia J per unit torque T is given by J=I/T, combining theprevious two equations (3) and (4) yields the following equation (5).

$\begin{matrix}{J = {{{\rho \cdot {t\left( {{b_{1} \cdot h_{1}} - {b_{2} \cdot h_{2}}} \right)}}y^{2}\text{/}{F\left( {y + c} \right)}} + {{\rho \cdot {t\left( {{b_{1} \cdot h_{1}^{3}} - {b_{2} \cdot h_{2}^{3}}} \right)}}\text{/}12{F\left( {y + c} \right)}}}} & (5)\end{matrix}$

Substituting, for example, b₁=25 mm, b₂=22 mm, h₁=22 mm, h₂=18 mm, t=5mm, c=10 mm, ρ=4.84·10⁻⁶ kg/mm³, and F=1N into the above equation (5)yields a graph as shown in FIG. 5.

It can be seen from FIG. 5 that, when the center of oscillation, i.e.,the oscillation axis X, is located on the outer side of the axis X₁ inFIG. 4A outside the coil 4 (on the upper side in FIG. 4A) (the positionwhere X=(h₁+h₂)/2=20 in FIG. 5), the value of J increases linearly. Inother words, when the oscillation axis X is located inside the coil 4,some parts of the coil 4 have long reaches and others have shortreaches. For example when the reach from the oscillation axis X to theaxis X₂ is long, the reach from the oscillation axis X to the axis X₁ isshort. This results in an increase in the inertia of one side of thecoil 4 along the axis X₂ and a decrease in the inertia of the other sideof the coil 4 along the axis X₁. However, if the oscillation axis X islocated outside the coil 4, all parts of the coil 4 have long reaches sothat the amount of the increase in inertia is greater than in the casewhere the oscillation axis X is located inside the coil 4. It can alsobe seen that the value of J increases steeply when the center ofoscillation, i.e., the oscillation axis X, is located closer to theposition where the thrust is produced (or the position of thrust) (whichcorresponds to the lower part in FIG. 4A and where X=0=X₂ in FIG. 5)than the axis X₀ in FIG. 4A (which corresponds to where X=X₁/2=10 inFIG. 5).

From the above, it is concluded that locating the oscillation axis X ofthe coil 4 between the axes X₀ and X₁ in FIG. 4A helps reduce the valueof the inertia per unit torque and accordingly lessen the influence ofthe inertia of the coil 4, thereby providing an oscillating part thatexecutes high-speed oscillatory motion.

As described so far, the voice coil motor in the pixel shifter 10(drive) according to the present preferred embodiment includes therectangular coil 4 producing thrust along the axis X₂ (on its one side),a flap 1 (oscillating part) fixing the coil 4, and the flat springs 2(elastic bodies) giving to the flap 1 an urging force against thethrust, wherein the oscillation axis X (the center of oscillation) ofthe oscillating part is parallel to the axis X₂ and located between thecoil face outside the axis X₁ (another side) that is parallel andopposite to the axis X₂, and the axis X₀ (centroid axis) that isparallel to the axis X₂ and passes over the centroid of the coil 4. Thisachieves high-speed oscillatory motion.

Second Preferred Embodiment

The pixel shifter 10 according to the first preferred embodiment canreduce the inertia J per unit torque by locating the center ofoscillation, i.e., the oscillation axis X, between the axes X₀ and X₁ inFIG. 4A. A second preferred embodiment defines an optimal range of thelocation of the center of oscillation, i.e., the oscillation axis X, bydetermining the range of the location of an oscillation axis X_(min)that exhibits a minimum value of the inertia J per unit torque.

The description is given with reference to FIGS. 4A and 4B shown in thefirst preferred embodiment. Like or corresponding components to those inthe first preferred embodiment are denoted by the same referencenumerals or characters as used in the first preferred embodiment, thedescription of which is thus omitted herein.

First, equation (5) given in the first preferred embodiment isdifferentiated with respect to y, in which case the values A and Bdefined by the following equations (6) and (7), respectively, shall beused for the sake of simplification.A=ρ·t(b ₁ ·h ₁ −b ₂ ·h ₂)/F  (6)B=ρ·t(b ₁ ·h ₁ ³ −b ₂ ·h ₂ ³)/12F  (7)

The differentiation of equation (5) with respect to y is thus expressedby equation (8), using the values A and B.dJ/dy=2A·y(y+c)⁻¹ −A·y ²(y+c)⁻² −B(y+c)⁻²  (8)

Sce the inertia J per unit torque attains its minimum value whendj/dy=0, both sides of equation (8) is multiplied by (y+c)², wheredj/dy=0. This derives equation (9).y ²+2c·y−B/A=0  (9)

From equation (9), the value y is given by equation (10).y=(1/2)·(−2c±√{square root over (4c ²+4B/A)})  (10)

The distance c between the axes X₀ and X₂ in equation (10) is given byequation (11), using the external height h₁ and the internal height h₂.c=(h ₁ +h ₂)/4  (11)

Substituting the value A given by equation (6), the value B given byequation (7), and the distance c given by equation (11) into equation(10) derives the following equation (12).

$\begin{matrix}{y = {{{- \left( {h_{1} + h_{2}} \right)}\text{/}4} \pm \sqrt{{\left( {h_{1} + h_{2}} \right)^{2}\text{/}16} + {\left( {{b_{1} \cdot h_{1}^{3}} - {b_{2} \cdot h_{2}^{3}}} \right)\text{/}\left( {12 \cdot \left( {{b_{1} \cdot h_{1}} - {b_{2} \cdot h_{2}}} \right)} \right)}}}} & (12)\end{matrix}$

Now, from equation (11), the ratio Y of y to c (i.e., Y=y/c) is given byequation (13).Y=y/((h ₁ +h ₂)/4  (13)

By substituting equation (13) into equation (12), the ratio Y can beexpressed by the following equation (14).

$\begin{matrix}{Y = {{- 1} \pm \sqrt{1 + \left( {4\left( {{b_{1} \cdot h_{1}^{3}} - {b_{2} \cdot h_{2}^{3}}} \right)\text{/}\left( {3\left( {h_{1} + h_{2}} \right)^{2}\left( {{b_{1} \cdot h_{1}} - {b_{2} \cdot h_{2}}} \right)} \right)} \right.}}} & (14)\end{matrix}$

If ((b₁·h₁ ³−b₂·h₂ ³)/(h₁+h₂)²(b₁·h₁−b₂·h₂))=Z in the above equation(14), Y=0 when Z=0. Assuming that Z≦1, when Z=1, the maximum value of Yis Y=−1+√{square root over (1+4/3)}≈0.53 (more precisely, less than0.53). Whether this assumption is correct or not is now discussed.

Referring to the value Z, since (h₁+h₂)²>0 and b₁·h₁−b₂·h₂>0, deformingthe expression giving the value Z with the assumption that Z≦1, weobtain the following inequality (15).b ₁ ·h ₁ ³ −b ₂ ·h ₂ ³≦(b ₁ ·h ₁ −b ₂ ·h ₂)·(h ₁ +h ₂)²  (15)

Further deformation of inequality (15) derives the following inequality(16).0≦h ₁ ·h ₂((h ₁ +h ₂)·(b ₁ −b ₂)+(b ₁ ·h ₁ −b ₂ ·h ₂))  (16)

Since it can be seen from FIGS. 4A and 4B that b₁−b₂>0 andb₁·h₁−b₂·h₂>0, the above inequality (16) holds true. That is, theabove-described assumption, Z≦1, is correct.

From the above, it is clear that the value of the ratio Y of thedistance y between the axis X₀ and the oscillation axis X to thedistance c between the axes X₀ and X₂ (=y/c) is within the range givenby the following inequality (17).0≦Y<0.53  (17)

This indicates that, when the distance from the axis X₀ to the axis X₁(which corresponds to the distance c between the axes X₀ and X₂) in FIG.4A shall be 1 and if another axis X₃ is located 0.53 away from the axisX₀ in the direction toward the axis X₁, the location of the center ofoscillation at which the inertia J per unit torque attains its minimumvalue is within the range from the axes X₀ to X₃.

As described so far, the voice coil motor in the drive according to thepresent preferred embodiment has the feature that, when the distancebetween one (axis X₂) or another (axis X₁) side and the centroid axis(axis X₀) shall be 1, the distance between the center of oscillation(the oscillation axis X) and the axis X₀ is less than 0.53. The secondpreferred embodiment, as compared with the first preferred embodiment,can thus further reduce the value of the inertia J per unit torque andaccordingly lessen the influence of the inertia of the coil 4, thusproviding the oscillating part that executes high-speed oscillatorymotion.

While the invention has been shown and described in detail, theforegoing description is in all aspects illustrative and notrestrictive. It is therefore understood that numerous modifications andvariations can be devised without departing from the scope of theinvention.

1. A drive comprising: a coil of a rectangular shape; a magnet disposedopposite to one side of said coil; and a yoke holding said magnet andpassing a magnetic line of force through said one side along with saidmagnet, wherein said coil passes a current, thereby producing thrust onsaid one side, said drive further comprising: an oscillating partholding said coil and having an oscillation axis that is parallel tosaid one side and is the center of oscillation; and an elastic bodygiving to said oscillating part an urging force against said thrust,wherein said coil has a centroid axis that is parallel to said one sideand passes over a centroid of said rectangular shape of said coil, saidcoil has another side that is parallel and opposite to said one side,said another side does not have a magnet and a yoke that pass a magneticline of force through said another side, and said oscillation axis islocated between a coil face outside said another side and said centroidaxis.
 2. The drive according to claim 1, wherein assuming that thedistance between said one or another side and said centroid axis is 1,the distance between said oscillation axis and said centroid axis isless than 0.53 in a direction from said centroid axis toward saidanother side.